However, the attack described by the FBI would be extremely hard to pull off, Todd said.

Most Asterisk systems are protected by firewalls or other security software and even if one could be accessed by a visher, administrators generally limit the number of calls any one account can make simultaneously, he explained. "Most of the time you would not be able to create thousands of calls in an hour."

The flaw affects older versions of Asterisk but not the most current version 1.6, he said.